One of the most challenging cover crop related issues is estimating the total plant available N and synchronizing N release with a subsequent cash crop from decomposing residues. The difficulty in estimation is due to the many variables that affect decomposition rates and N mineralization such as: residue quality, edaphic soil properties, environmental conditions, biological activity, and farm management activities (Baggie et al. 2004; Cambardella and Elliott 1993; Goh and Tutuna 2004; Johnson et al. 2007; Robertson and Groffman 2007). Improving our ability to predict N availability from cover crop residues requires further understanding of the interactions between biotic and abiotic variables as well as identifying measurable parameters that can reflect and/or predict N cycling in the soil. This knowledge will assist agricultural professionals in becoming more efficient nutrient managers by adopting and tailoring cover crop management practices to meet farm goals and environmental conditions.

Research

Materials and methods:

The field experiment was located at the Center for Environmental Farming Systems (CEFS) located in Goldsboro, North Carolina during 2010. The field trial was conducted at two different locations at CEFS approximately 3 mi apart from each other. Both sites had organic management but for different lengths of time. They will be referred to as “site 1” and “site #2”. Five warm-season cover crop treatments comprised of regionally popular selections were included: 1) buckwheat (Fagopyrum esculentum), 2) sorghum-sudangrass (Sorghum bicolor X S. bicolor var. sudanese (Special Effort)), 3) cowpea (Vigna unguiculata (Iron &amp; Clay) (L.) Walp.), 4) German foxtail millet (Setaria italica (L.) P. Beauv.), 5) German foxtail millet/cowpea mix, and 6) bare-ground controls. The design was a randomized complete block with multiple warm-season cover crop treatments. Treatments were replicated four times. Crop management and data collection were carried out by block. Each plot was 40 x 13 ft2. The soil at both locations was a Wickham sandy loam.

Field preparation was conducted during the month of April. One month later, each field was prepped for cover crop planting with a chisel plow and field conditioner. A drill seeder was used to plant cover crop seeds. All cowpea seeds were treated with Bradyrhizobium sp. (Vigna) inoculant prior to planting. The length of the growing season was different at each site due to poor crop establishment at the initial site #2 field location. Therefore, site #2 was re-planting at an adjoining location after the application of pre-plant amendments aimed at increasing soil nitrogen (i.e., soybean meal) and sulfur (K2SO4) levels. Periodic hand-weeding and/or shallow tillage with a tractor-mounted rototiller was carried out in the bare-ground control plots at both sites as needed to minimize living vegetation throughout the experimental time frame.

Irrigation was used during crop establishment and dry spells during the growing period however, the capability to irrigate at site #1 was more limited. Irrigation was not used during the post- cover crop incorporation sampling period. Buckwheat did not perform well at either location and therefore was dropped from the experiment at both sites. The remaining cover crops were terminated by cutting with a flail mower during the month of August. Crop residue was allowed to desiccate on the soil surface for 1 week before the cover crop residue was incorporated into the soil approximately 12 in deep with 2 passes from a tractor-mounted rototiller followed by a cultipacker.

Non-replicated air temperature, soil temperature and soil moisture were monitored on an hourly basis at each of the two field sites with automatic data loggers. Gravimetric soil moisture was calculated for each plot on each sample date. Precipitation data and the 30-year average for air temperature from 1971-2000 were supplied by the North Carolina State Climate Office weather station located 1 mi from the research station. Overall, the 2010 summer cover crop growing season was hotter and drier than average; additionally, there was a moderate drought period followed by record-breaking rainfall during the month of September (State Climate Office of NC 2010).

Immediately prior to termination, the percent cover crop cover and above-ground biomass was assessed. A sub-sample of dry plant tissue was ground and analyzed for elemental carbon and nitrogen. Another subsample was analyzed for dry matter and ash along with the sequential determination of neutral detergent fiber (NDF), acid detergent fiber (ADF), hemicellulose, cellulose, and lignin (Van Soest et al., 1980).

Soil samples were collected over a series of 6 sampling dates including, the day before cover crop termination and 5 dates post- cover crop incorporation. At each date, 40 soil cores from each plot were gently mixed together to create one homogenized composite sample. Within 48 hours of field collection, soil samples were hand-sieved to pass through a 2 mm sieve. Half the sieved soil was dried while the other half was kept field-moist and stored at 40°F. Subsamples from the dried soils were ground and analyzed for elemental C and N as described previously for plant tissue.

Soil carbon decomposition and nitrogen mineralization were determined via a 28 day aerobic incubation experiment. Soils were adjusted for optimal moisture and temperature during a 5 day pre-incubation period. Then at day 0 and day 28, soil samples were removed, mixed with 1 M KCl and analyzed for inorganic N via flow injection methods with colorimetric determination. The difference between the inorganic nitrogen before (day 0) and after incubation (day 28) was estimated to be the potential N mineralization. A vial containing NaOH was also added to each incubation unit at day 0 to absorb carbon dioxide (CO2-C) from the present soil sample. The amount of CO2-C evolved was estimated by titration with HCl at day 11 and day 28 (Zibilske 1994). The sum of the CO2-C evolved was estimated to be the potential C mineralization.

Two methods were used to measure inorganic soil N. Inorganic soil N was extracted from soil samples at the 6 sampling dates with 1 M KCl and analyzed for inorganic N via flow injection methods with colorimetric determination. In addition, plant root simulator probes (PRS) were used to measure in situ nitrogen availability. The probes consisted of double-sided ion exchange resins that were pre-treated to adsorb either the nitrate (NO3-N) or ammonium (NH4-N) ions. Four pairs of PRS-probes were buried in the top 0-15 cm of soil. Sets of PRS probes were buried and retrieved 5 times over approximately 11 weeks post-cover crop incorporation; they were not buried pre-incorporation. Protocols for burial, retrieval, washing, handling and shipping provided by the supplier were followed (Western Ag. Innovations, Inc., SK, Canada). Adsorbed ions (NH4-N + NO3-N) were extracted from the probe membranes and analyzed for inorganic nitrogen (NH+4-N + NO-3-N) via flow injection methods with colorimetric determination.

A density fractionation method was used to separate the free, particulate organic matter (F-POM) fraction (?1.6 g cm-3) from soil samples (adapted from Marriot and Wander, 2006). F-POM was then subjected to a combustion-based elemental analysis, as described previously, to determine N and C content.

Colormetric techniques were used to assess cellulose degradation with both ?-1,4-glucosidase (BG) (EC 3.2.1.21) and cellulose 1,4-?-cellobiosidase (CBH) (EC 3.2.1.91) activity and lignin degradation with peroxidase (PER ) (EC 1.11.1.7) activity. Enzyme activity rates were determined by incubating soil with standard concentrations of p-nitrophenyl (pNP) bound substrates estimated to be above soil potential soil enzyme saturation levels, and then measuring the amount of pNP released (Parham and Deng, 2000; Turner et al., 2002). The protocols for BG and CBH were very similar; only the substrate type and incubation time differed between the two 96-well plate assays (Marx et al., 2001; Saiya-Cork et al., 2002). The resulting products were estimated by light absorbance (i.e., optical density) at 410 nm. Peroxidase activity was determined by incubating soil with the 3,3′,5,5′-tetramethylbenzidine substrate and subsequently measuring the oxidized reaction product (Johnsen and Jacobsen, 2008). The reaction products were estimated by light absorbance (i.e., optical density) at 525 nm. An extinction coefficient of 0.059 ?M cm-1 was used in peroxidase calculations.

All cover-cropped soils subjected to a 28-day aerobic incubation were found to have positive mean potential nitrogen mineralization. Cowpea-dominated crops had greater potential for nitrogen mineralization compared to both grasses. Even cover crops with C:N ratios >40:1 were predicted to result in net nitrogen mineralization. Based on our data, we hypothesize that dry soil conditions affected microbial N demand by adjusting (i.e., lowering) their carbon use efficiency (CUE). The potential N mineralization of cover crops was strongly correlated with shoot tissue quality. Further investigation into residue biochemical composition revealed significant differences between cowpea-dominated and grass cover crops for neutral detergent fiber, hemicellulose and lignin:N ratio.

It has been well-documented that cool-season cover crops can retain soil N, moderate N release and reduce nitrate leaching over the fall to early spring seasons (Brinsfield and Staver 1991; Dabney et al. 2001; De Vos et al. 2000Lewan 1994; MacDonald et al. 2005), our data demonstrated that warm-season cover crops can provide these functions at different points of the year. At the pre-incorporation date, cover crop plots had lower extractable soil N compared to bare-ground controls, reflecting plant uptake of soil N during the summer growing season. At our sites, warm-season cover crops appeared to retain and subsequently moderate the release of soil N by converting it from inorganic to organic forms during the growing season. For approximately 4-6 weeks after cover crop termination (i.e., 2-4 weeks post- incorporation), sorghum-sudangrass plots in particular, had lower inorganic soil N values compared to bare-ground soils and the legume-dominated plots; the differences among cover crop types appeared to be driven by the amount of crop biomass rather than cover crop quality (e.g. C:N ratio).

PRS probes appeared to be an informative measure of in-situ inorganic soil N. Because the technology relies on the movement of N via mass flow or diffusion in the soil over time it was able to reflect the effects of soil moisture on hypothetical plant-available nitrogen in a different manner than traditional point-in-time soil extracts. Changes to inorganic soil N status after the record-breaking precipitation event were reflected by increased ion adsorption by the PRS probes indicating a flush of inorganic N had occurred. The utility of PRS probes for appears promising but may depend on frequency and length of deployment intervals.

Overall, we did not find F-POM to be a good indicator of potential N mineralization as results were inconsistent across our two sites. However, F-POM C:N was positively correlated with the cover crop shoot C:N. In addition, the F-POM C:N of legume-dominated plots was lower than grass plots indicating that F-POM reflected the properties of the cover crop residues in the weeks to months after soil incorporation. The incorporation of cover crop residues stimulated an increase in both mean soil microbial biomass N and cellulase enzyme activity as hypothesized. Of the 3 enzymes that we examined, ?-1,4-glucosidase activity was the most sensitive to differences among cover crop quality and was positively correlated with both mineralized soil C and N. Our results support the growing opinion that biological soil indicators can provide sensitive assessments of the impacts and benefits of agricultural management practices such as cover cropping.

Educational & Outreach Activities

Participation Summary

Education/outreach description:

Additional presentations and publications from this work are anticipated in the future.

North Carolina State University, Horticultural Science Dept. Seminar, The great cover-up: Searching for indicators of short-term nitrogen mineralization from cover crops in organic farming systems. April 2012.

Project Outcomes

Project outcomes:

Our data suggested the following applications to farm management practices. Legume cover crops, as expected, appeared to have more N mineralization potential then grasses but during a dry year the differences were not as appreciable. In years or systems when soil moisture is not a limiting factor, net N mineralization may occur at a faster rate compared to a dry year and agricultural professionals may want to take this into consideration in regards to applying supplemental fertilizer to cash crops. We found evidence which demonstrated that warm-season cover crops can help conserve and protect soil N resources thereby making agricultural systems less susceptible to nutrient losses. When regions are susceptible to intense precipitation events (e.g., hurricane season) or susceptible to leaching, the utilization of high biomass cover crops appeared to maximize the ability to protect soil inorganic N. In these situations, retention of soil nitrogen by cover crops can provide both a cost-savings as well as environmental benefit (e.g., protect water quality). Federal and state programs which provide incentives for the use of cool-season cover crops should extend these benefits to include warm-season cover crops.

Farmer Adoption

Evaluation of farmer adoption was not part a goal of this study but some practical implications of this research are described in the Impact of the Results/Outcomes section.

Recommendations:

Areas needing additional study

Replicating this field study at additional sites and over multiple years would help refine our conclusions. Additional research addressing the effect of soil moisture on cover crop decomposition and short-term nitrogen mineralization would help establish some guidelines for farmers to consider based on seasonal weather occurrences. Focusing plant residue quality studies on soluble nitrogen fractions as well as further exploration of neutral detergent fiber and hemicellulose concentrations may offer insight into the subtle differences among relatively high quality plant residues. Further evaluation of the predictive capacity of the cellulase enzyme, ?-1,4-glucosidase in relation to cover crop residue decomposition and short-term net N mineralization appears to be a valuable pursuit.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.

Primary Sidebar

Footer

1122 Patapsco Building | University of Maryland | College Park, MD 20742-6715

This site is maintained by SARE Outreach for the SARE program and features research projects supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture. SARE Outreach operates under cooperative agreement award No. 2018-38640-28731 with the University of Maryland to develop and disseminate information about sustainable agriculture. USDA is an equal opportunity provider and employer.